1. Field of the Invention
The present invention relates to the white balance for a camera and, more particularly, to an auto white balance apparatus and an auto white balance method in a digital camera with a strobe.
2. Description of Related Art
It is known that the color of an object is decided by two factors: the selective reflecting spectrum and the light source spectrum of the environment that the object locates. Thus, one object can exhibit different colors when light illuminating onto the object is different from the previous one. Conventionally, a white balance device is provided for a digital camera to make the captured image exhibit consistent color in environments with light sources of different color temperatures.
For detecting or estimating the light source of the environment, a number of techniques have been proposed. A simple one is to completely rely on the strobe apparatus in a camera. Once the strobe apparatus flashes, the spectrum of the flash is taken as a standard, and the pre-stored value in memory is taken as the setting value of white balance. A detail description of such can be found in U.S. Pat. No. 5,001,552, which is incorporated herein for reference. With this technique, because no other light sources are taken into consideration and the color components of the strobe apparatus is known, it is easy to achieve the white balance.
However, unless it is in a dark room, light may be reflected from other light sources and enters into the camera. As a result, the white balance effect is not satisfactory due to over-simplifying the components of color temperature.
With reference to FIG. 1, there is shown an improved white balance technique. At first, an image is captured before the strobe apparatus flashes, and the brightness Y0 of the image is determined. Next, the strobe apparatus flashes and another image is captured, and the brightness Y1 of the image is determined. By comparing the brightness Y0 and Y1, the ratio of the flash and the other light sources can be known, so as to adjust and set the white balance device. Finally, the strobe apparatus flashes and reaches a predetermined level, and an image is captured again and the captured image is adjusted based on the new white balance setting. A detail description of such can be found in U.S. Pat. No. 5,568,194, which is incorporated herein for reference. Although this technique can achieve a better white balance effect, its process is much more complicated. For example, it requires flashing for two times, and capturing image for three times, and the R/G/B signals of the whole image must be converted into Y signals prior to performing brightness comparison. Moreover, this technique only works based the following two assumptions. First, the time interval between any two of the three image-capturing operations must be very short so that the influence from light source except the strobe apparatus can be deemed as a constant. Second, the brightness of two consecutive flashing operations must be the same so that the comparison between the image captured under the first flash and the image captured with flash is meaningful. However, because the flashing operation consumes lots of energy, the main capacitor of the strobe apparatus typically requires a long time to be charged for making the brightness of two consecutive flashing operations the same, if no additional light source is provided, and this will contradict the first assumption. If the time interval is reduced, the second assumption will be contradicted due to the fact that the main capacitor can not be sufficiently charged.
Therefore, it is desirable to provide a novel auto white balance apparatus and method in a digital camera with a strobe to mitigate and/or obviate the aforementioned problems.
The object of the present invention is to provide an auto white balance apparatus and an auto white balance method in a digital camera with a strobe, which can utilize a flash module to obtain information related to the light of flash lamp and the other light source, thereby accurately adjusting the white balance setting.
In accordance with one aspect of the present invention, there is provided an auto white balance apparatus in a digital camera with a strobe, which comprises: an image sensor for capturing external images; an image processing circuit having an auto exposure control circuit for setting an exposure time based on brightness of the image captured by the image sensor; and a flash module including a flash lamp for providing light source while taking a picture, a photo detector for detecting brightness when the flash lamp flashes and converting the detected brightness into a first voltage signal, and a comparator for comparing the first voltage signal with and a reference voltage signal, so as to stop flashing the flash lamp when the first voltage signal is larger than the reference voltage signal, wherein, before the flash lamp flashes, the photo detector detects brightness based on the exposure time set by the auto exposure control circuit for establishing a second voltage signal, so as to perform a white balance adjustment based on a ratio of the reference voltage signal and the second voltage signal.
In accordance with another aspect of the present invention, there is provided an auto white balance method in a digital camera with a strobe. The digital camera has an image sensor for capturing external images and a flash module. The flash module stops flashing when a first voltage signal representing a detected brightness of the flash module is larger than a reference voltage signal. The method comprises the steps of: (A) determining an exposure time based on brightness of the image captured by the image sensor; (B) detecting brightness based on the exposure time by the flash module for establishing a second voltage signal; and (C) performing a white balance adjustment based on a ratio of the reference voltage signal and the second voltage signal.
Other objects, advantages, and novel features of the invention will become more apparent from the detailed description when taken in conjunction with the accompanying drawings.